In the related art, a technology is known which accurately detects phase current values of each phase so as to control switching elements of a bridge circuit which rotatably drives a multiphase electric motor, using PWM control.
For example, JP-A-2013-219905 discloses a control device of a multiphase rotary machine which appropriately secures a current detection time, while stabilizing control of a three-phase rotary machine. The control device includes a shunt resistor as current detection means between a lower MOS of an inverter unit and a negative electrode of a battery. While lower MOS's of two phases are turned on and a lower MOS in the other one phase is turned off, among three phases, the shunt resistor detects currents flowing through shunt resistors in the two phases in which the lower MOS's are turned on. The control device estimates a current in the one phase in which the lower MOS is turned off, using the Kirchhoff's law, and calculates a duty updating value during an updating period which is half a calculation period of a duty command value, by using linear interpolation based on the duty command value. In a case where the duty updating value of any one phase exceeds a maximum duty corresponding to a shortest detection time required for current detection which is performed by the shunt resistor, the duty updating value of each phase is increased. Thereby, while the lower MOS's in the two phases except for the phase are turned on, the current detection is performed, and thus, it is possible to appropriately secure time for the current detection which is performed by the shunt resistor.
In addition, JP-A-2010-220414 discloses a motor control device which ensures current detection with high accuracy and increases a voltage utilization ratio. In a case where an ON time of any one of respective switching elements on a low potential side corresponding to each phase is shorter than a detection time of a current value, in a drive circuit, the motor control device estimates a phase current value of an undetectable current phase, based on current values of two phases except for the undetectable current phase corresponding to the switching element. In addition, when the current detection is performed by using the estimation, the motor control device holds a switching state of a switching arm corresponding to the undetectable current phase, while the current detection of the two phases except for the undetectable current phase which is a basis of the estimation is performed. That is, since a high potential side switching element is maintained to be turned on and a low potential side switching element is maintained to be turned off, the motor control device outputs a motor control signal which prevents noise that is generated by the switching operation from being mixed thereinto. Thereby, although an output voltage limitation which is set to secure a detection time of phase current values of all phases is abolished, the motor control device can ensure current detection with high accuracy, and as a result, it is possible to increase the voltage utilization ratio.